The ammonium salts of fluorinated alkanoic acids are generally used in the aqueous emulsion polymerization of fluorinated olefin monomers such as tetrafluoroethylene, chlorotrifluoroethylene or vinylidene fluoride because of their high surface activity and low rate of telogen formation. In the course of these polymerizations, some of the fluorinated olefins hydrolyze to produce fluoride ions. After polymerization is complete and the resulting fluoropolymer is isolated, the fluorinated alkanoic acid ammonium salt is present in a dilute aqueous solution, along with ionic fluorides and other impurities. It is highly desirable to recover these fluorinated alkanoic acids and their salts for reuse, both because of their high cost and the need to minimize pollution problems. For such reuse, any impurities in the recovered compounds that may inhibit or change the course of the subsequent polymerization must be removed. Of particular use in such applications are the ammonium salts of fluorinated alkanoic acids, such as, for example, perfluorooctanoic acid (PFOA).
A known method of purification is by acidifying a fluorinated alkanoic ammonium salt solution by adding sulfuric acid until a pH of 1 to 2 is obtained, and subjecting the acidified solution to steam distillation. This is an energy-inefficient process, and yields a dilute solution of about 10-16 wt % fluorinated alkanoic acid. This must be neutralized with ammonia and concentrated for reuse by a means such as reverse osmosis. In an additional difficulty, the fluorides in the solution can co-distill during the steam distillation and corrode the distillation equipment.
U.S. Pat. No. 4,609,497 discloses a process for the recovery of ammonium salt of PFOA from a solution containing a selected nonionic polyether by extracting the ammonium salt with an organic liquid, decanting the organic layer containing the PFOA, contacting it with alumina to adsorb the PFOA, contacting the alumina with ammonium hydroxide to remove the PFOA, acidifing and decanting the resulting solution, steam distilling the organic phase in the presence of an acid and an oxidizing agent, and neutralizing the PFOA with ammonium hydroxide. This process is cumbersome to carry out, and requires the addition and removal of an organic extractant and an alumina adsorbent.
U.S. Pat. No. 4,282,162 discloses a process wherein, after a fluorinated carboxylic acid has been recovered from an aqueous solution by adsorption on a basic ion exchanger, it is eluted using a mixture of a mineral acid and an organic solvent. The eluate is then decanted, the lower organic layer neutralized and acidified, and the precipitated fluorinated carboxylic acid filtered. The fluorinated carboxylic acid obtained by this process is not of polymerization grade. U.S. Pat. No. 5,312,935 discloses a process for purifying such acid by oxidizing it in a solution containing less than 9% by weight of water. This process requires special ion exchange equipment, frequent replacement of the ion exchange resin, the addition and removal of an organic solvent, and a further purification step.
U.S. Pat. Nos. 5,442,097 and 5,591,877 disclose a process for recovery of fluorinated carboxylic emulsifiers from an aqueous solution containing its salt by acidification of the salt, reacting the acid with an alcohol to form the ester, distilling the resulting ester mixture, and decanting the ester layer. The resulting ester may be reacted with aqueous ammonia to form the corresponding ammonium salt. This process requires introduction of an organic reactant, which must be recovered, and requires carrying out additional esterification and de-esterification reactions.
The above processes are complicated and expensive to carry out, requiring multiple conversion steps in which the fluorinated alkanoic compound is either transferred to other media, such as an organic solvent or adsorbing solid, or converted to an organic ester, followed by transfer or conversion back to the fluorinated alkanoic compound. There is a need to develop a simpler process in which the fluorinated alkanoic acid can be isolated by an energy-efficient means, and without such transfers or conversions. An advantage of the invention is that the invention process does not require transferring the fluorinated alkanoic acid to an organic solvent or an adsorbent, or its conversion to an organic ester. Another advantage of the invention is it does not require concentration of the purified ammonium salt solution with reverse osmosis, and it minimizes the content of corrosion-causing fluorides.